1. Field of the Invention
The present invention relates to power supply for a cold cathode fluorescent lamp (CCFL) system, and more particularly to an apparatus for driving each cold cathode fluorescent lamp (CCFL) in a multiple CCFL system.
2. Prior Art
Fluorescent lamps are used in a number of applications where light is required but the power required to generate light is limited. One such application is the backlight for a notebook computer or similar electronic device. One popular type of fluorescent lamp is a cold cathode fluorescent lamp (CCFL). This lamp typically requires a high starting voltage (about 1500 volts) for a short period of time, in order to ionize the gas contained within the lamp tube and thereby ignite the lamp. After the gas is ionized and the lamp is ignited, less voltage is needed to keep the lamp on.
FIG. 4 shows a conventional CCFL driving apparatus 1. The driving apparatus 1 includes a power supply 11, a buck pre-regulator 12, a self-resonating circuit 13, a CCFL 14, a resistor 15, a capacitor 16, a feedback circuit 17, and a pulse width modulation circuit 18. The CCFL 14, the capacitor 16, and the resistor 15 formed a closed loop.
The power supply 11 supplies a voltage to the buck pre-regulator 12, which regulates an operation current in the driving apparatus 1. The self-resonating circuit 13 receives the input current signal regulated by the buck pre-regulator 12, and outputs a high voltage (about 1500 volts) to start the CCFL 14. After that, the self-resonating circuit 13 outputs a lower voltage (about 600 volts) to maintain the CCFL in a steady illuminated state. The feedback circuit 17 receives a current signal from the closed loop, and feeds the current signal back to the pulse width modulation circuit 18. An output of the pulse width modulation circuit 18 is coupled to the buck pre-regulator 12, to provide a pulse width modulation signal thereto in order to modulate the duty cycle thereof.
The driving apparatus 1 also can be used to drive a multiple CCFL system, in which it is important to have balanced currents in all lamps in the system. The driving apparatus 1 provides a feedback circuit 17 to evenly distribute current through each of the CCFLs. However, in practice, characteristics of the CCFLs in the multiple CCFL system are not exactly the same as each other. Therefore, even when the operation currents in the CCFLs are equal to each other, there are still differences in the brightnesses between the various CCFLs. Furthermore, as the CCFLs randomly age after prolonged use, the differences in the brightnesses between the various CCFLs become even more pronounced.
Therefore, it is desirable to provide an improved driving apparatus which overcomes the above-described disadvantages of conventional driving apparatuses.